Gabriele Sandri, a 26-year-old disc jockey from Rome, was sitting in a car that had stopped at a highway rest area near Arezzo in Tuscany when he was fatally shot in the neck by a bullet fired by a police officer.

Arezzo police chief Vincenzo Giacobbe said a policeman was being investigated for possible manslaughter.

On Sunday, police in Arezzo said that the officer, from a position on the other side of the highway, had fired two warning shots in the air to break up a scuffle at the rest stop. But on Monday, they gave a different version of what they have called a "tragic error."

National police chief Antonio Manganelli promised to get to the truth of the shooting, criticizing the policeman for an "inept" action.

Initial reports said police intervened to stop a scuffle between Sandri's group of Lazio fans -- headed to see their club play at Inter Milan -- and a group of Juventus fans, also traveling north to see their team play.

On Monday, authorities raised doubts that fan rivalry had fueled the fight.

"[The officer] intervened to calm down what appeared to be a fight," Giacobbe said. "He didn't know if they were fans. Another thing to clarify is why it was called a fight between fans. We think so, but [the officers on the scene] didn't know it. They saw a fight beginning, yelling, banging on a car."

On Monday, Giacobbe said the fatal shot apparently was fired "horizontally," across four lanes of highway.

Milan daily Corriere della Sera quoted the policeman as saying the second shot went off accidentally.

Sandri's death forced the postponement of two Serie A matches and the suspension of another as clashes erupted in cities including Milan and Bergamo. Enraged by the shooting, rioters smashed windows and hurled stones at police cars.

In the capital, violent fans rioted into the night, setting trash bins and police vans on fire.

About 75 police officers were injured in Rome, police said. The Italian Olympic Committee -- attacked along with nearby police barracks -- estimated damage to its headquarters at about 100,000 euros (US$147,000).

Late on Monday, the soccer federation said it would suspend games set for Sunday, when Serie B and C play. The Serie A isn't scheduled to play this weekend because the national team has a match against Scotland.

A national watchdog body for soccer violence announced on Monday that large groups of violent fans would be blocked from traveling to certain games. The monitoring body already labels games by risk level, and visiting fans will be barred from traveling to high-risk matches until a system of fan identity cards can be instituted.

Also, by March 1, all stadiums with a capacity of more than 7,500 will have to provide match stewards, the monitoring body announced. Otherwise, the games will be held behind closed doors.